1. Field of the Invention
This invention relates to a printer for executing printing by an electrophotographic process, and more particularly, to an electrostatic recording system using dielectric belt which can be suitably used for color printing.
2. Description of the Related Art
Electrophotographic color printing methods for printing a plurality of toner images having mutually different colors such as black, yellow, magenta and cyan, on a sheet of paper in superposition can be broadly classified into the following two kinds.
The first kind is a system or method which uses four color developing devices for one photoconductor drum or belt, develops and transfers toner images, each color one-by-one, and repeats this procedure for each color, in other words, four times in all. Transfer methods from the photoconductor drum or belt to the sheet of paper in this case include a method which transfers the toner to the sheet of paper through an intermediate transfer belt or drum. Another transfer method transfers the toner to the sheet of paper without using such an intermediate member. In either of these methods, the transfer operation must be repeated four times to one sheet of paper, and they suffer from the drawback that the printing speed drops to 1/4 to that of the case where the transfer is made once. However, these transfer methods have been widely employed in the past for compact and economical electrophotographic printing apparatuses.
The second kind is a so-called "tandem" printing system or method which sequentially aligns four developing devices and four photoconductor drums for the four colors and prints the color image by the single conveying operation of the sheet of paper. Transfer of the toners from the photoconductor drums to the sheet of paper is carried out for the respective photoconductor drums while the sheet of paper passes them once, and the sheet of paper is conveyed to a fixing device after the transfer of the four colors, and the toners are thereafter fixed to the sheet of paper.
The present invention relates to a recording system which carries out conveying of the sheet of paper and transfer of the toners to the sheet of paper by using a transfer belt.
An example of a prior art method using a transfer belt is shown in FIG. 1. In FIG. 1, reference numeral 1 denotes a photoconductor drum, reference numeral 2 denotes a transfer belt, reference numeral 3 denotes a sheet of paper, reference numeral 4 denotes a corona electrifier for attraction, reference numeral 5 denotes an electrifying brush, reference numeral 6 denotes a corona electrifier and reference numeral 7 denotes an electrifier for deelectrification. The sheet of paper 3 is conveyed by the transfer belt 2. At the entrance portion to the photoconductor drum 1, both the transfer belt 2 and the sheet of paper 3 are electrified by the corona electrifier 4 disposed below the transfer belt 2 and the electrifying brush 5 disposed above the transfer belt 2, respectively, so that the sheet of paper 3 is attracted to the transfer belt 2. At the exit portion of the sheet of paper 3, the charge of the sheet of paper 3 is removed by the corona discharge from the electrifier 7 for deelectrification, and the sheet of paper 3 is separated from the transfer belt 2.
According to this method, both the sheet of paper 3 and the transfer belt 2 are inevitably electrified to the same polarity. Therefore, in order to prevent the sheet of paper 3 from being wound up by the photoconductor drum 1 and to execute the transfer of the toner from the photoconductor drum 1 to the sheet of paper 3 while the sheet of paper 3 is kept attracted to the transfer belt 2, the sheet of paper 3 must be electrified to the same polarity as that of the surface of the photoconductor drum 1.
When the surface of the photoconductor drum 1 is electrified to a negative polarity, for example, the corona electrifier 4 at the sheet entrance portion applies an electrifying voltage of a positive polarity so as to electrify the surface of the transfer belt 2 and a sheet of paper 3 to the negative charge when the voltage is applied from the back of the transfer belt 2. Therefore, the transfer voltage by the corona electrifier 6 must be elevated and consequently, the attractive force between the sheet of paper 3 and the transfer belt 2 becomes so high that, when the sheet of paper 3 is separated from the transfer belt 2, the charge of both the transfer belt 2 and the sheet of paper 3 must be removed by the electrifier 7 for deelectrification.
Because deelectrification by the electrifier 7 must be conducted by the corona discharge having an opposite polarity to that of the sheet of paper 3 and the toner, the toner on the sheet of paper 3 is attracted and scattered by the corona electrifier 7 for deelectrification, thereby lowering the image quality. Accordingly, a transfer method which can prevent the sheet of paper 3 from being wound into the photoconductor drum and can improve separability by lowering the transfer voltage has been desired.